1. Field of the Invention
In optical communications systems a communications signal is propagated along an optical transmission line, such as an optical fibre or other waveguide by modulation of one or more parameters of an optical carrier. Such transmission lines are expensive to lay and equipment used at their terminations or at repeaters to amplify the optical signals along the transmission line add to this expense. Optical fibre transmission lines have been installed widely during the last few years but already demand for lines is exceeding the capacity of those transmission lines so that it is now necessary to install more transmission lines to provide additional capacity or to increase the capacity of the existing optical transmission lines.
2. Description of the Related Art
One way of increasing the capacity of existing lines is to establish bi-directional transmission over a single optical fibre transmission line. Advantageously, duplex transmission may be used, transmitting data in both directions simultaneously using the same bandwidth. In practice, this is achieved by splicing directional optical couplers onto both ends of a fibre. The directional optical couplers sort the signals from the transmission line in accordance with their direction of travel. Modifying an optical transmission line to take advantage of duplex transmission requires very little investment, typically only a few hundred dollars and is therefore very much cheaper than the provision of a new line.
Another way in which the capacity of an existing optical transmission line can be increased is to increase the rate of data or information transmission along the line and use multiplexing techniques such as time division multiplexing or wavelength division multiplexing, to transmit two or more channels of information simultaneously in the same direction over the same optical transmission line. Time division multiplexing is particularly efficient as many streams or channels of data or information can be interleaved with one another at the transmission end of the line, transmitted over the line at high speed and then, at the downstream end of the line, demultiplexed and separated into their separate channels. Naturally, additional equipment is required to carry out multiplexing and demultiplexing operations but the cost of this equipment is still much less than the cost of laying a corresponding number of new optical fibre transmission lines.
Whilst these techniques for increasing the capacity of an optical fibre transmission line are possible, they normally require the use of expensive hardware to overcome the problems and limitations associated with them. Accordingly with conventinal systems these techniques have not led to the advantages that might, theoretically, have been gained.
With bi-directional transmission techniques, problems can arise in detecting faults in the line such as a break in a waveguide or a failure of a repeater or a far end transmitter. If the waveguide breaks at least some of the light being transmitted in one direction along the waveguide from one end may be reflected and returned to the one end. Usually, insufficient transmission takes place across the broken ends of the fibre to allow one end of the transmission line to receive information transmitted from the other end. However, if the light transmitted in the one direction is reflected at the break the one end of the transmission line may still receive a channel of information of sufficient quality for this channel of information to appear to supervisory and error detecting circuits associated with the one end to be a completely normal channel of information. When the transition takes place between the information transmitted from the other end and that reflected from the one end, normally errors are detected and line decoders and any demultiplexers lose alignment. However within a short period, provided that the information reflected from the break is of sufficient quality, the line decoders and demultiplexers regain alignment and appear to be operating correctly. Thus, after the transient interruption no fault is detected at either end of the optical transmission line even though no information is being transmitted directly from one end of the other, or vice versa. The transient detected by supervisory circuits and any detected malfunction of supervisory telemetry is likely to be interpreted as a supervisory fault since the system is apparently working correctly and supervisory systems are often unreliable.
Reflective fault conditions may arise in a number of ways. For example, if a fibre is broken the two ends at the break may be highly reflecting. Also, faults at transmitters, receivers, repeaters, couplers, connectors and other system components may reflect or couple light back along the waveguide. Reflections from connectors may give rise to fault identification problems if there is also a transmitter failure at the far end from the receiver, especially over a short line. Normally reflections from connectors particularly at the distant end are small compared with the signal transmitted from that far end, and do not significantly impair the quality of signal transmission. In the event of a significant reduction in the magnitude of the signal transmitted from the other end, the reflected signal may become dominant. If the quality of the received reflected signal is sufficiently good then after some transient change-over effects the receiver and associated performance monitors interpret this reflected signal as a valid channel of information, and again this is unlikely to be identified.
Couplers may also, under certain circumstances, couple light so that it is detected at the originating end.
In this specification, the term `reflective fault condition` is intended to cover any situation where there is reflection or coupling of signal or data or any other fault which results in data being received at the end from which it orginated.